Gallium and indium alkoxides are useful as precursors to their respective oxides, in isolation or combined with each other or in combination with tin and/or zinc alkoxides to form amorphous oxide semiconductors employed in thin film photovoltaic devices. The chief advantage of alkoxides over other chemical forms of the eponymous elements is low processing temperature by the sol-gel technique especially in its non-hydrolytic application (elimination of ester and/or ether condensation products). Display components fabricated by less convenient means have already been demonstrated (Nomura et al, 2006, Jpn J Appl Phys 45(5B) 4303-8).
Methods of synthesis of gallium and indium alkoxides have been reviewed by Carmalt & King (2006 Coordination Chemistry Reviews 250, 682-709).
Probably the best-known synthetic method is the reaction of the metal tri-halide with 3 equivalents of sodium alkoxide, NaOR (where R is the desired alkyl chain). This method was first taught for gallium alkoxide by Mehrotra & Mehrotra (1964 Current Sci. (India) 33, 241) and by Funk & Paul (1964 Anorg Allg. Chem. 330, 70), and subsequently for indium alkoxide by Chatterjee et al, (1976 J. Ind. Chem. Soc. 53, 867). Other methods are known, but they employ expensive reagents and lack scope for economy of scale upon commercial scale-up.
It was recently revealed (U.S. Pat. No. 6,426,425) that gallium and indium alkoxides prepared in this way are contaminated with residual chloride and such ionic impurities are terminally detrimental to the operation of subsequently fabricated electronic devices. U.S. Pat. No. 6,426,425 discloses a process for purifying gallium alkoxide (i.e. removing the chloride contaminant) by adding potassium alkoxide followed by distillation or sublimation. The resulting product has a significantly reduced level of chloride contamination, for example 200 ppm.
The present invention aims, in one object, to provide a method of synthesising an alkoxide of gallium or indium, from the corresponding halide, in such a way as to substantially avoid the presence of any contaminating halide, and thereby avoid the need for further expensive processing stages.
It is a further object to provide a method of synthesising a metal alkoxide from the corresponding halide, in such a way as to substantially avoid the product being contaminated with sodium or potassium. The synthetic method of the invention provides a particularly pure product without the need for any further reactions.